| Viral synergism is caused by co-infection of two unrelated viruses,leading to more severe symptoms or increased titres of one or both viruses.Polerovirus is one of the most economically important taxa of plant viruses.Synergistic infection of phloem-restricted poleroviruses and umbraviruses has destructive effects on crop plants.The mechanism underlying this synergy remains elusive.In this thesis,biological,transcriptomic and small RNA profiling analysis were conducted to understand synergistic interaction between a polerovirus Brassica yellows virus(BrYV)and an umbravirus Pea enation mosaic virus 2(PEMV 2)on Nicotiana benthamiana.Synergism was observed in co-infections of BrYV and PEMV 2 on N.benthamiana,which led to(1)appearance of severe symptoms;(2)increased titres of BrYV;(3)gain of mechanical transmission capacity of BrYV;(4)broader distribution of BrYV to non-vascular tissues.BrYV PO mutant viruses,namely BrYV-APO and BrYV-POH10Q/T11P,cannot synergistic interact with PEMV 2 on N.benthamiana.In contrast,synergism occurred between a BrYV read through domain(RTD)-deficient mutant(Br△RTD)and PEMV 2.Furthermore,transgenic N.benthamiana plants expressing BrYV P0-6xMyc or PEMV 2 ORF4-3×Flag were obtained for further study.Our results revealed the synergistic characteristics during co-infection of BrYV and PEMV 2.Transcriptomic analysis showed that a total of 1778 differentially expressed genes were unique to BrYV+PEMV 2 infection,including genes involved in photoynthesis,ethylene,plant defense,RNA silencing,transcription factors and protein translation.We speculated that these genes contained potential host factors involved in viral synergism.Profiles of virus-derived small interfering RNAs(vsiRNAs)from BrYV and PEMV 2 in singly and doubly infected plants were obtained by small RNA deep sequencing.Our results showed that accumulation of BrYV vsiRNAs increased tremendously and ratio of positive to negative strand BrYV vsiRNAs differed between singly infected and co-infected plants.Positions to which the BrYV vsiRNAs mapped to the viral genome varied considerably during synergistic infection.In addition,14 differentially expressed miRNAs that were unique to BrYV+PEMV 2 infection were identified(6 upregulated and 8 downregulated).Our results revealed possible effects of synergism have on vsiRNAs and miRNAs.In a separate project,the genome sequence of a new polerovirus that infects field pea and faba bean in China was determined,and the name Pea mild chlorosis virus was proposed.Moreover,The complete genomic sequences of two distinct Beet western yellows virus(BWYV)genotypes infecting sugar beet in Beijing,namely BWYV-BJA and BWYV-BJB,were determined.Sequence analysis showed that BWYV-BJA and BWYV-BJB shared 93.0%nucleotide sequence identity,with relatively high variability within 5’-terminal half of genome and high conservation within other regions.Phylogenetic analysis and recombination analysis suggested that BWYV-BJ and BWYV-BJ might be recombinant viruses. |
